Dynamic motion spinal stabilization system

Abstract

Provided is a system for dynamically stabilizing a spine. In one example, the system includes a first bone anchor coupled to a first polyaxial head and a second bone anchor coupled to a second polyaxial head. An axis passing through a center of each polyaxial head is aligned with a center of rotation. A first member has a first end movably coupled to the first polyaxial head and a second end. A second member has a third end coupled to the second polyaxial head and a fourth end moveably coupled to the second end. The first and second members are configured to maintain the alignment of the axes with the center of rotation during three dimensional movement of the first member relative to the second member.

1. A dynamic stabilization system for attachment to vertebral bodies, said dynamic stabilization system comprising:
 a first bone anchor coupled to a first polyaxial head and a second bone anchor coupled to a second polyaxial head, wherein a first longitudinal centroidal axis passing through a center of said first polyaxial head is aligned with a center of rotation and a second longitudinal centroidal axis passing through a center of said second polyaxial head is aligned with said center of rotation, wherein said center of rotation is within a preselected A-P mark range of said vertebral bodies when said dynamic stabilization system is attached to said vertebral bodies, and wherein said first bone anchor is alignable independently of said first longitudinal centroidal axis and wherein said second bone anchor is alignable independently of said second longitudinal centroidal axis; 
 a first member having a first end movably coupled to the said first polyaxial head and a second end; and 
 a second member having a third end coupled to said second polyaxial head and a fourth end moveably coupled to said second end, wherein after said dynamic stabilization system is attached to said vertebral bodies said first and second members are configured to maintain the alignment of said first and second longitudinal centroidal axes with said center of rotation during three dimensional movement of said first member relative to said second member. 
 
     
     
       2. The dynamic stabilization system of  claim 1  further comprising a first bearing post coupling said first end and said first polyaxial head, wherein said first bearing post is rotationally coupled to said first end and immovably coupled to said first polyaxial head, and wherein a first longitudinal axis of said first bearing post intersects said center of rotation. 
     
     
       3. The dynamic stabilization system of  claim 2  wherein said first bearing post locks a position of said first bone anchor relative to said first polyaxial head. 
     
     
       4. The dynamic stabilization system of  claim 2  wherein said third end is immovably coupled to said second polyaxial head. 
     
     
       5. The dynamic stabilization system of  claim 2  further comprising a second bearing post coupling said third end and said second polyaxial head, wherein said second bearing post is rotationally coupled to said third end and immovably coupled to said second polyaxial head, wherein a second longitudinal axis of said second bearing post intersects said center of rotation, and wherein said first and second members are configured to maintain said alignment of said first and second longitudinal axes with said center of rotation during three dimensional movement of said first member relative to said second member. 
     
     
       6. The dynamic stabilization system of  claim 5  wherein said first bearing post is inserted into a first aperture of said first end and said second bearing post is inserted into a second aperture of said third end. 
     
     
       7. The dynamic stabilization system of  claim 6  further comprising first and second bushings inserted into said first and second apertures, respectively, wherein each of said first and second bushings includes a threaded bore configured to engage threads of said first and second bearing posts, respectively, and wherein each of said first and second bushings rotates freely with respect to said first and second apertures, respectively. 
     
     
       8. The dynamic stabilization system of  claim 7  further comprising first and second locking caps securing said first and second bearing posts, respectively, to said first and second bushings. 
     
     
       9. The dynamic stabilization system of  claim 1  further comprising a pin coupling said first and second members, wherein said pin is inserted into an aperture in each of said second and fourth ends, and wherein said pin forms a pivot point around which said first and second members rotate relative to one another. 
     
     
       10. The dynamic stabilization system of  claim 9  wherein said second end includes first and second arms forming a yoke, wherein said fourth end fits between said first and second arms, and wherein said pin extends through said first and second arms. 
     
     
       11. The dynamic stabilization system of  claim 1  further comprising a third member coupling said first and second members, wherein said third member has a fifth end movably retained in a cavity in said second end and a sixth end movably retained in a cavity in said fourth end. 
     
     
       12. A dynamic stabilization device for attachment to vertebral bodies comprising:
 a first member having first and second ends, wherein said first end is rotatably coupled to a first polyaxial head about only a first axis that extends through said first end and intersects a center point, wherein said center point is within a preselected A-P mark range of said vertebral bodies when said dynamic stabilization system is attached to said vertebral bodies and wherein said first sol axial head is configured to be head fixedly coupled to a bone anchor; 
 a second member having a third end and a fourth end, said third end rotatably coupled to a second polyaxial head about only a second axis that extends through said third end and intersects said center point, and said fourth end moveably coupled to said second end, wherein said second polyaxial head is configured to be fixedly coupled to a bone anchor; and 
 a third member movably coupling said second and fourth ends, wherein after said dynamic stabilization system is attached to said vertebral bodies said first, second, and third members are configured to maintain the intersection of said first and second axes with said center point as said center point moves along a curved three dimensional surface during movement of said first member relative to said second member. 
 
     
     
       13. The dynamic stabilization device of  claim 12  wherein said third member is a pin that rotationally couples said first and second members, wherein said pin is received by a first aperture in said second end and a second aperture in said fourth end. 
     
     
       14. The dynamic stabilization device of  claim 12  wherein said third member includes a first portion slideably received by a first aperture in said second end and a second portion slideably received by a second aperture in said fourth end. 
     
     
       15. The dynamic stabilization device of  claim 14  further comprising a retaining means for retaining at least one of said first and second portions in said first and second apertures, respectively. 
     
     
       16. The dynamic stabilization device of  claim 12  wherein said first end includes an aperture configured to receive a bearing post. 
     
     
       17. The dynamic stabilization device of  claim 16  further comprising a bearing element retained in said aperture, wherein said bearing element includes a threaded bore configured to engage threads of said bearing post and wherein said bearing element rotates freely with respect to said aperture. 
     
     
       18. The dynamic stabilization device of  claim 12  wherein said third end is configured to movably couple to said second polyaxial head. 
     
     
       19. The dynamic stabilization device of  claim 12  wherein said first and second members each have a curvature defined to maintain said intersection of said first and second axes with said center point as said center point moves along said curved three dimensional surface. 
     
     
       20. A method comprising:
 identifying a center of rotation between first and second vertebrae; 
 coupling a first member of a dynamic stabilization device to a first polyaxial head; 
 coupling a second member of said dynamic stabilization device to a second polyaxial head; 
 orienting an axis of said first polyaxial head with said center of rotation; 
 orienting an axis of said second polyaxial head with said center of rotation; and 
 securing said first and second members relative to said first and second polyaxial heads, respectively, to maintain the orientation of said first and second axes with said center of rotation during movement of said first member relative to said second member. 
 
     
     
       21. The method of  claim 20  wherein securing said first member relative to said first polyaxial head includes locking a height of said first member relative to said first polyaxial head. 
     
     
       22. The method of  claim 20  further comprising inserting a first bone anchor coupled to said first polyaxial head and inserting a second bone anchor coupled to said second polyaxial head. 
     
     
       23. A dynamic stabilization system for attachment to vertebral bodies, said dynamic stabilization system comprising:
 a left construct comprising a left upper polyaxial screw head attaching to an upper vertebra, a left lower polyaxial screw head attaching to a lower vertebra, a left first member connected to said left upper polyaxial screw head, and a left second member connected to said left lower polyaxial screw head, said left first member and said left second member being hingedly connected to each other; and 
 a right construct comprising a right upper polyaxial screw head attaching to said upper vertebra, a right lower polyaxial screw head attaching to said lower vertebra, a right first member connected to said right upper polyaxial screw head, and a right second member connected to said right lower polyaxial screw head, said right first member and said right second member being hingedly connected to each other; 
 wherein a first longitudinal centroidal axis passing through a center of said left upper polyaxial head is aligned with a center of rotation, a second longitudinal centroidal axis passing through a center of said left lower polyaxial head is aligned with said center of rotation, a third longitudinal centroidal axis passing through a center of said right upper polyaxial head is aligned with said center of rotation, and a fourth longitudinal centroidal axis passing through a center of said right lower polyaxial head is aligned with said center of rotation; 
 wherein said left first member and said left second member have a first concave curvature with respect to said vertebral bodies when installed on said vertebral bodies, said first concave curvature defined to maintain said intersection of said first longitudinal centroidal axis and said second longitudinal centroidal axis with said center of rotation; and 
 wherein said right first member and said right second member have a second concave curvature with respect to said vertebral bodies when installed on said vertebral bodies, said second concave curvature defined to maintain said intersection of said third longitudinal centroidal axis and said fourth longitudinal centroidal axis with said center of rotation.